As global energy consumption increases, the amount of fossil fuels that are used as an energy source is gradually increasing. The use of fossil fuels causes climate change and environmental pollution, which have come to the fore as global problems. In order to solve these problems, attempts have been made to efficiently utilize electric power using renewable energy and energy storage systems.
Here, renewable energy, excluding energy based on coal, petroleum, nuclear energy and natural gas, refers to solar energy, biomass, wind power, small hydro power, fuel cells, coal liquefaction, gasification, marine energy, waste energy and others, and also indicates liquid fuel made out of byproducts of geothermal heat, hydrogen and coal, but is substantially an energy source able to replace petroleum. Various kinds of renewable energy are advantageous because no environmental pollution occurs and energy development is possible, but suffer from poor quality of energy due to variation in output depending on the geographical conditions and the natural environment. With the goal of solving such problems, an energy storage system that is capable of storing the remaining power and then supplying it flexibly when it is needed is emerging as the most suitable means, and in particular, a large-capacity long-term storage system is receiving attention.
Among a variety of energy storage systems, a redox flow battery is a secondary battery that employs oxidation and reduction of a redox couple or an active material dissolved in an electrolyte solution, unlike existing secondary batteries, in which electric energy is stored in an electrode containing an active material. A redox flow battery is configured such that a stack responsible for output and an electrolyte solution unit responsible for capacity are separately disposed, whereby the capacity and output may be freely designed. Only the oxidation-reduction reaction occurs through electron transfer at the electrodes in the redox flow battery, unlike other batteries, and thus there is no structural change of the electrode itself, and the electrode and the active material are separated from each other, whereby side reactions do not take place between these two materials, thus realizing high stability and a long lifetime compared to other kinds of secondary batteries. The electrolyte solution, which is one of key materials of the redox flow battery, is used in a manner in which active materials having different oxidation states are dissolved in a water-soluble or water-insoluble solvent. Here, various type of redox flow battery is formed depending on the kind of active material, and aqueous and non-aqueous electrolytes are provided depending on the kind of solvent. The electrolyte solution containing the active material has to have high reactivity with the electrode and reversibility, and also has to have a wide potential window and high solubility in order to increase energy density. The electromotive force of the redox flow battery is determined by the difference in standard electrode potential Eº of the redox couple that constitutes a cathode electrolyte solution and an anode electrolyte solution, and examples of main aqueous redox couples developed to date include Fe/Cr, V/V, V/Br, Zn/Br, Zn/Ce, etc.
Meanwhile, development of a redox flow battery for space engineering using a Fe/Cr-based active material began in NASA (National Aeronautics and Space Administration), USA, in 1974. Initially used as the active material of the redox flow battery, Fe/Cr is problematic in terms of permeation of the active material through the separator and corrosion with the electrolyte, and thus the use thereof is limited.
Since then, many researchers have studied redox couples that have excellent stability and enable reversible electrochemical reactions with the electrodes. In 1980, a vanadium redox flow battery was developed by Maria Skyllas-Kazacos et al. of Australia. Vanadium, having various oxidations states, may be utilized as a single material for both the cathode and the anode. When vanadium is used as a single active material, even in the case that the battery capacity is decreased due to permeation of the vanadium active material through the separator, the battery capacity can be restored through rebalancing. However, the water-soluble vanadium active material suffers from low voltage and solubility and thus low energy density. In order to increase the energy density, when the concentration of the vanadium active material is increased, vanadium is precipitated on the anode V (II, III) at a low temperature of 0° C. or less, and vanadium pentoxide (V2O5) is precipitated on the cathode at an operating temperature of 40° C. or higher. In the case where precipitates are generated in the flow battery system, flow of the electrolyte solution is interrupted, and thus the inner pressure of the stack is increased, causing leakage, and the battery capacity is decreased due to the vanadium precipitate. Furthermore, with regard to vanadium, which is used as the active material of the flow battery, China possesses 40% of the world's reserves and thus large price fluctuations occur in response to changes in export volume, and moreover, patent technology regarding a vanadium active material and a flow battery using the same is held in foreign countries, and the use thereof is thus restricted.
[Citation List] Korean Patent Application Publication No. 10-2014-0016298